Why was Texas divided after it joined the USA? (Short Animated Documentary)

When Texas joined the USA in 1845 it was about 50% larger than it was today but five years after it had joined, its northern and western lands had been handed over to the federal government. But given that Texas had fought a war with Mexico for this land why did it give it up and why did the US feel the need to take it? To find out watch this short and simple animated history documentary.